President Trump sent another shock wave through the legal system Friday with an abrupt Justice Department demand that all forty-six U.S. Attorneys held over from the Obama Administration hand in their resignations immediately. The U.S. Attorney from the storied Southern District of New York, Preet Bharara, had good reason to think he might be exempt from this demand: in November, following a highly publicized meeting in Trump Tower, then-President-elect Trump asked Mr. Bharara to stay on.

What changed? Mr. Bharara simply may have been a victim of raw power politics—he is a former chief counsel to Senator Chuck Schumer and there is no love lost between the Senate minority leader and the president. But as I noted in November, the SDNY is no ordinary crime-fighting post. It’s been home to law-enforcement legends such as Henry Stimson, Thomas Dewey, Robert Morgenthau and Rudy Giuliani. It’s size, Manhattan location and jurisdiction over Wall Street make it an important player in the U.S. justice system, famously independent of Washington.

Mr. Bharara established himself as a respected non-partisan scourge of political corruption, sending to jail more than twenty-five corrupt New York political figures, including Speaker of the New York State Assembly Sheldon Silver, a Democrat, and the majority leader of the New York State Senate, Dean Skelos, a Republican. The SDNY is currently taking a close look at associates of New York City Mayor Bill DeBlasio and Governor Andrew Cuomo in separate corruption cases.

The SDNY is connected to several Trump-related matters as well. It’s deep into a complex series of tax and money-laundering cases involving Deutsche Bank, a major Trump Organization creditor. It’s investigating whether pro-Trump Fox News structured illegal payments to employees in sexual harassment cases. An ethics group filed a lawsuit against President Trump in the Southern District alleging he has violated the Emoluments Clause of the Constitution. Mr. Trump has been caught up in controversies involving Russia and Turkey, and Mr. Bharara has tangled with both countries. He is banned from Russia over his successful prosecution of international arms dealer Victor Bout and has been condemned by the president of Turkey for making an Iran sanctions-busting case against the Turkish gold trader Reza Zarrab.

Mr. Bharara refused to hand in his resignation on Friday. On Saturday, the Justice Department fired him. On Sunday, he noted in a cryptic tweet, “now I know what the Moreland Commission must have felt like.” Governor Cuomo created the Moreland Commission in 2013 to investigate public corruption in New York and suddenly shut it down a year later, infuriating Mr. Bharara. He investigated the closure for possible obstruction of justice but did not bring charges.

Was Mr. Bharara signaling with his tweet that he too had been shut down with nefarious intent? U.S. Attorneys serve at the pleasure of the president and it is standard practice for an incoming administration to install its own people. But rarely does it happen in such a sweeping and abrupt manner.

Some of us with long memories remember the events of early 1993, when press reports were mounting about federal criminal investigations related to the Whitewater land deal and the new president, Bill Clinton. His Attorney General, Janet Reno, fired every sitting U.S. Attorney in the land. A longtime Clinton associate, Paula Casey, took over as U.S. Attorney for the Eastern District of Arkansas, rejecting early criminal referrals on Whitewater from the Resolution Trust Corp. and turning down a plea bargain overture from a crooked Arkansas insider before recusing herself from the case.

That’s how you derail an investigation. When the Senate takes up President Trump’s nomination for the next U.S. Attorney for the Southern District, it should make sure the candidate is a person of the caliber of a Preet Bharara or a Robert Morgenthau, independent and free from any taint of cronyism.